The United Nations is expected to place sanctions on four British jihadists accused of fighting or recruiting for the Islamic State group in Syria, the British government said on Tuesday.

The two women and two men would face a travel ban and an asset freeze in a bid to discourage others leaving Britain to join the jihadist group, which has seized control of large areas of Syria and Iraq.

"We will do all we can to stop British citizens from going to fight for ISIL," a spokesman for Prime Minister David Cameron said, using another term for Islamic State.

At least 700 Britons are thought to have travelled to join extremists in the region where IS has been extending its influence amid the chaos caused by the devastating civil war in Syria that has killed 240,000 and displaced millions.

The British government said it was the first time in almost a decade it had asked the UN to add Britons to its Al-Qaeda sanctions list of jihadists, adding that it could ask for more people to be included.

The women were named as 21-year-old Asqa Mahmood, a former student suspected of leaving Britain to operate in a religious enforcement unit in the IS stronghold of Raqqa in Syria.

The other is Sally Anne Jones, 46, a former rock musician dubbed the "punk jihadi" by the British tabloids, who is accused of recruiting women online after travelling to Syria with her husband.

Also named were former supermarket security guard Abu-Said al-Britani, or Omar Hussain, 28 -- who told the BBC in 2014 he would only return to Britain to "plant a bomb" -- and former medical student Nasser Muthana, 21, who has appeared in IS propaganda.

Cameron is meeting US President Barack Obama and other leaders to coordinate opposition to IS.

Britain has joined a US-led coalition of Western and Arab allies conducting air strikes against IS in Iraq, but not in Syria, as parliament refused to sanction Cameron's plan to join military action there in 2013.

World leaders on Monday stepped up efforts at the UN General Assembly in New York to find a solution to restore peace to Syria, which has been torn apart by the violence of various armed groups and the regime of Bashar al-Assad, causing a severe refugee and humanitarian crisis.